Two Vastly Different Aspects of Submarine Warfare and Mercury

"Submarines have long been used for special operations - carrying commandos, reconnaissance teams, and agents on high-risk missions. Most special operations by US submarines are carried out by SEALs, the Sea-Air-Land teams trained for missions behind enemy lines. These special forces can be inserted by fixed-wing aircraft, helicopter, parachute, or surface craft, but in most scenarios only submarines guarantee covert delivery. 

Once in the objective area, SEALs can [travel from submarine then near to shore by SEAL Delivery Vehicle (SDV) to] carry out combat search-and-rescue operations, reconnaissance, sabotage, diversionary attacks, monitoring of enemy movements or communications, and a host of other clandestine and often high-risk missions. 

US nuclear powered submarines have repeatedly demonstrated the ability to carry out special operations involving many swimmers. During exercises, which include Army, Air Force, and Marine Corps special operations personnel as well as SEALs, submarines recover personnel who parachute from fixed-wing aircraft and rappel down from helicopters into the sea, take them aboard, and subsequently launch them on missions. These Special Warfare Team Missions include combat swimmer attacks, reconnaissance and surveillance, infiltration and exfiltration across the beach, beach feasibility studies, hydrographic survey, and Surf Observation Teams in support of amphibious landing operations." (Source FAS org).

And then there is nuclear deterrence or mutually assured destruction via Trident II SLBM (below):

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Launches can also be peaceful - like the launch of John Glenn as part of Project Mercury. Australia, including the lights of Rockingham, feature from 4 mins, 35 secs in.

Pete